A flash memory is a kind of PROM (Programmable ROM) possible for electrically re-writing data. The flash memory utilizes a single transistor made to perform both a program input function of an EPROM (Erasable PROM) and an erasing function of an EEPROM (Electrically Erasable PROM) by combining advantages of the EPROM, in which a memory cell is composed of a single transistor so that the area of the memory cell is small, and data can be collectively erased through ultraviolet radiation; and the EEPROM, in which data can be electrically erased, while a memory cell is composed of two transistors so that the area of the memory cell is larger. This type of flash memory is referred to as a Flash EEPROM.
Since memory information is not erased even though a power source is turned off, such a flash memory is referred to as a non-volatile memory, and it is different from DRAM (Dynamic RAM), SRAM (Static RAM) or the like in that it is non-volatile.
Flash memories may be classified into a NOR-type structure in which cells are arranged in parallel between a bit line and a ground and a NAND-type structure in which cells are arranged in series. A NOR-type flash memory with a parallel structure has been widely used for booting a cellular phone because a high-speed random access is possible when performing a reading operation, and a NAND-type flash memory with a serial structure has an advantage in that it is suitable for storing data and is profitable for miniaturization because a reading speed is slow but a writing speed is fast. Further, the flash memories may be classified into a stack gate type and a split gate type depending on the structure of a unit cell, and classified into a floating gate device and a SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) device depending on the shape of an electric charge storage layer.
The floating gate device among these devices generally includes a floating gate made of polycrystalline silicon surrounded by an insulator, and an electric charge is injected into or discharged from the floating gate through channel hot carrier injection or Fowler-Nordheim tunneling so that storing and erasing data are accomplished.
FIG. 1 illustrates a sectional view of a semiconductor substrate with a floating gate array formed thereon in a process of manufacturing a flash memory device. The section of the substrate shown in FIG. 1 shows a vertical section parallel to a word line of the flash memory device. In the conventional flash memory device, a series of device isolation films 11, e.g., STIs (Shallow Trench Isolations), are formed in a direction perpendicular to the word line on the substrate 10 so as to define an active device region. Thereafter, a silicon oxide film 12 used as a tunnel oxide film is formed in a predetermined thickness on the entire surface of the substrate 10, and a polycrystalline silicon layer to be used as a floating gate is then formed on the silicon oxide film 12. The polycrystalline silicon layer formed in such a manner is patterned through photo and etching processes to form an array of a plurality of floating gates 14.
The respective plurality of floating gates 14 formed in such a manner constitute one memory cell in which the neighboring floating gates 14 are spaced apart from one another at an interval W. Since the floating gates 14 are generally patterned through photo and etching processes, it is not easy to reduce the interval W by any significant amount. Further, the method of manufacturing the conventional flash memory device is progressed through a process of first forming STIs on a substrate and then patterning floating gates through an additional photo process as described above, and a minimal alignment margin should be secured in order to prevent the misalignment of an etching mask in the process of forming the floating gates. Accordingly, since the interval of the floating gates needs to be maintained larger than a certain size, the integration of a device cannot be further reduced by relying upon the conventional method of manufacturing the flash memory. Further, since very expensive equipment needs be used in a case of the photo process, the manufacturing costs of the products is increased.